1. Field of the Invention
The present invention relates to an apparatus for recording information as a hologram by using interference between two light beams and, more particularly, to an information recording apparatus which can perform multiplexed recording of high-quality holograms on a moving medium.
2. Description of the Related Art
A rewritable optical disk as an information recording apparatus has been increasingly used mainly as an external storage medium of a computer system, in place of a magnetic disk and the like. Such an optical disk has a recording density of about 5.times.10.sup.5 bit/mm.sup.2 as its basic performance. That is, a 5-inch optical disk has a storage capacity corresponding to 3,300 pages of newspapers.
With advances in information communication techniques, however, demands have arisen for information storage techniques allowing high-rate data transfer with higher density. Especially in the field of image communication services proposed as a future service vision, an ult6ra-high-speed (1 Gbit/sec), high-density (10.sup.8 bit/mm.sup.2) storage system is required to handle storage of a large of a large amount of information of, e.g., high-resolution color motion images, multi-screen images, and stereoscopic images, and high-speed retrieval of data from databases. In order to handle such operations, studies are currently undertaken in various institutions to increase the recording density by means of a short-wavelength laser and to increase the data transfer rate by means of multiple beams. With regard to the recording density, however, even if a recording/reproduction scheme using a short-wavelength laser is realized, it is expected that the density limit is about 10 times that of a currently used optical disk at best. In addition, since the recording principle is so-called thermal recording based on heating/cooling processes of a medium using radiation of a laser beam, the recording density is also limited by thermal interference between bits. With regard to the transfer rate, a great improvement in performance cannot be achieved in principle in a conventional bit-by-bit recording/reproduction scheme because of limitation in the number of multiple beams.
As described above, in a storage system using conventional optical disks, a great improvement in performance, in terms of recording density and data transfer rate, cannot be expected. In order to overcome such limitations, a new storage scheme must be established, which can achieve an increase in recording density by means of multiplexed recording and an increase in transfer rate by means of collective processing of a plurality of bits.
The basic arrangement of an apparatus for collective recording/reproduction of two-dimensional digital information by a holographic recording scheme is disclosed in, e.g., L. d'Auria, J. P. Huignard, C. Slezak, and E. Spitz, "Experimental Holographic Read-Write Memory Using 3-D Storage", APPLIED OPTICS, vol. 13, No. 4, April 1974, pp. 808-810.
According to this study, a laser beam is diffracted by an acoustooptic element capable of two-dimensional beam deflection. The primary diffracted beam is split into two beams in two directions by a beam splitter. One beam is collimated by a collimator lens and is subsequently addressed to a specific lens of a lens array. In a page composer, a two-dimensional bit pattern constituting one-page information is formed. The beam spread by the addressed lens of the lens array is radiated on the page composer to be formed into a signal beam. The signal beam is focused on a point on a holographic recording medium by a Fourier transform lens. The other beam formed by the beam splitter is superposed, as a reference beam, on the focus position of the signal beam on the recording medium by an electrooptic element and a holographic diffraction grating. In this manner, the two-dimensional digital information formed by the page composer is recorded as a minute hologram.
In order to form a hologram at a different position on the recording medium, the primary diffracted beam is deflected in a direction different from that mentioned above by using the acoustooptic element. As a result, the minute holograms are arranged on the stationary recording medium in the form of a two-dimensional matrix. In addition, by deflecting only the reference beam using the electrooptic element, multiplexed recording of a hologram having different information can be performed at the position of the already recorded hologram.
In reproduction, when the minute holograms are accessed by the reference beam used in recording, the holograms are collectively reproduced, and the two-dimensional bit pattern formed by the page composer is detected by a detector array.
In such a scheme, however, a complicated optical system is required to access an arbitrary hologram and convert a reference beam angle in angle multiplexed recording. Therefore, a high-speed, high-precision access of a beam is difficult to perform. In addition, since there is no exchange function of recording media, the storage capacity is limited by the number of resolvable spots of an acoustooptic deflector or an array lens number.